The present invention relates to through-type kilns and methods of operating same for firing molded ceramic articles, particularly bricks, wherein the ceramic articles are conveyed successively through preheating, firing and cooling zones of the kiln while an opposed counter-current air flow cools the ceramic articles within the cooling zone and heats the ceramic articles within the preheating zone.
So-called tunnel kilns are utilized in brick making plants for firing bricks on a generally continuous basis. The structure of such kilns defines a substantially linear tunnel through which the ceramic articles pass in succession in a substantially linear path of travel through a preheating zone into a firing zone and therefrom through a final cooling zone. Typically, such tunnel kilns may be of a length of between 100 and 200 meters. The ceramic articles to be fired are supported on a plurality of individual kiln cars connected end-to-end in a train and are preliminarily passed through a dryer before entering the preheating zone. Within the preheating zone, the ceramic articles are gradually subjected to increasing temperature as they travel to the firing zone, to minimize any possible cracking of the ceramic material resulting from dramatic temperature changes. Within the firing zone, the ceramic articles are subjected to intense heat which may reach temperatures of 1000.degree. Centigrade or more. After firing, the ceramic articles are gradually cooled by passage through the cooling zone to reach a temperature of approximately 120.degree. Centigrade before exiting the tunnel kiln.
The firing zone is preferably heated by gas-fueled burners. To facilitate the cooling and preheating of the ceramic articles within the respective cooling and preheating zones, a forced air current is directed into the tunnel kiln adjacent the exit end of the cooling zone to flow therefrom forwardly through the cooling, firing and preheating zones in opposition to the direction of travel of the ceramic articles through the tunnel kiln. In this manner, the relatively cool air entering the cooling zone aids in cooling the ceramic articles within such zone, the air gradually increasing in temperature as it flows through the cooling and firing zones into the preheating zone wherein the heated air assists in preheating the ceramic articles entering the kiln. Within the preheating zone, the heat exchange between the air current and the relatively cooler ceramic articles entering the kiln serves to re-cool the air before it is exhausted from the kiln via a chimney typically located at the entrance to the preheating zone.
In order to maintain the counter-current air flow as abovedescribed, it is necessary to maintain a pressure gradient through the tunnel kiln from the exit end of the cooling zone to the entrance end of the preheating zone. Since the air flow used for cooling and subsequently for preheating the ceramic articles is not recycled in a conventional tunnel kiln, it is common that approximately three kilograms of air are required for every kilogram of the ceramic articles to be fired. As will be readily recognized, a substantial amount of potential energy is lost in this process and, further, a significant negative impact on the environment results. If the exhausted air must be purified of gases and other substances released into the air current during the firing process, filter systems must be provided with a capacity to handle the air flow through the tunnel kiln.
It is known to recover a portion of the cooling air from the cooling zone and a portion of the heat energy from the air exhausted through the chimney of the tunnel kiln for use otherwise in the overall manufacturing operation. In brick making plants, the recovered energy is typically utilized to heat dryers for freshly formed bricks.
West German Patent Application S 21,776, filed Feb. 2, 1951 and published for opposition on June 26, 1952, discloses a through-type kiln wherein a preheating zone, a calcining zone and a cooling zone are arranged in succession to one another for conveyance therethrough of material to be treated. To provide cooling of the material in the cooling zone and heating of the material in the preheating zone, a counter-current of air is blown through the kiln in opposition to the direction of material conveyance but, in order to avoid unnecessary removal of heat from the calcining zone, the air current bypasses the calcining zone and flows directly from the cooling zone to the heating zone through the use of structural covers or closures arranged at opposite sides of the calcining zone. As an alternative to such a cover or closure system this application suggests that the kiln may be constructed to cause the material being treated to turn -80.degree. while passing through the calcining zone from the preheating zone to the cooling zone so that the preheating and cooling zones are arranged directly adjacent one another to facilitate direct conveyance of the air flow from the cooling zone to the preheating zone. However, as will be understood, it would be highly impractical to attempt to utilize this kiln construction in a brick making kiln since the kiln cars fully loaded with bricks may weight several tons which, together with the temperatures of 1000.degree. Centigrade or more prevailing in the firing zone, would present substantial difficulties in designing an economical means for accomplishing this manner of operation in a brick making kiln.